There is a lot of confusion about image resolution — what is DPI? What is PPI? How do you calculate them and why do they matter?
Here’s a case-study:
Let’s say you want to print a digital image to a size 4×6 inches. What should be the dimensions for your digital image?
The first thing to know is that 300 DPI (dots-per-inch), is the standard “print-quality” resolution: let’s work backwards from there.
To print an image at a size of 4 x 6 inches at 300 dots per inch, you would need:
300 dots (per inch) x 4 inches wide = 1200 dots for the width
300 dots (per inch) x 6 inches high = 1800 dots for the height
Consider it now in digital terms: the dot is now called a “pixel” (hence PPI = pixels-per-inch), and you’ve already reached your first requirement: The image needs to be 1200 pixels wide and 1800 pixels high (see Sidenote 1 for info about Megapixels).
The next thing to consider is the point density.
For example, let’s take your 1200×1800 image. How do you tell it to print as a 4×6 image, rather than an 8×10 image? That’s where DPI/PPI comes in. The standard resolution used for internet images is 72dpi (see Sidenote 2 for more info about the 72dpi standard).
Let’s say you were to print it at that “web-standard” resolution of 72 dpi; this means that for each inch-long printed line, you’ll print 72 dots. Considering you have 1200×1800 dots at your disposal, you can calculate that the image will print at:
1200 pixels (or dots) / 72 pixels per inch (or dots per inch) = 16.67 inches wide
1800 pixels (or dots) / 72 pixels per inch (or dots per inch) = 25 inches high
Although image will therefore be much larger than the target of 4×6 inches, it won’t look good. 72dpi is not enough to trick the eye into believing that it’s a continuous image; if you print an image at 72 dpi, it will look pixelated. The industry-standard for “print-quality” is 300dpi: literally, 300 dots stuffed into one inch of linear space.
And this is the second half of the equation: you need to set your image resolution to 300dpi to tell the system how to print your 1200×1800 image.
Now, with your 1200×1800 pixel image at 300dpi, you can calculate that it will print at:
1200 pixels wide / 300 pixels per inch = 4 inches wide
1800 pixels high / 300 pixels per inch = 6 inches wide
For more depth and illustrated examples, here’s a great article that discusses image resolution and print quality in more depth. Take a look and become an expert!
Also, be sure to check our our Resources page for useful links and forms. We’ll keep updating it throughout the year:
Sidenote 1: Megapixels
How do the pixel dimensions relate to the concept of “Megapixels”?
Let’s go back to our digital image dimension 1200 pixels wide and 1800 pixels high. What is the surface area of a rectangle?
Height x Width = 1200 x 1800 = 2,160,000 pixels.
Next, by the metric standard, “Mega” means million. This means that 1,000,000 pixels = 1 megapixel. Now you can derive that 2,160,000 pixels = 2.16 Megapixels (MP).
In other words, to take a photo that will print at 4×6 inches at standard “print-quality”, you need a digital camera that can handle at least 2.16 MP.
For comparison’s sake, the iPhone 5′s camera has a resolution of 8MP.
Sidenote 2: The 72DPI Standard
The 72DPI standard for digital images is actually kind of a false measurement, because what matters is the pixel density (or resolution) of your display. For example, a 13-inch laptop screen may have the same display resolution (let’s say 1024 x 768 pixels) as a 21-inch monitor—so the same image will appear larger on the 21-inch monitor.
This is a whole other interesting story which is beyond our scope here, but we encourage you to look it up!